During construction of oil and gas wells, a rotary drill is typically used to bore through subterranean formations of the earth to form a borehole. As the rotary drill bores through the earth, a drilling fluid, known in the industry as a “mud,” is circulated through the borehole. Drilling fluids are usually pumped from the surface through the interior of the drill pipe. By continuously pumping the drilling fluid through the drill pipe, the drilling fluid can be circulated out the bottom of the drill pipe and back up to the well surface through the annular space between the wall of the well bore and the drill pipe.
Once the wellbore has been drilled, casing is lowered into the wellbore. A cement slurry is then pumped into the casing and a plug of fluid, such as drilling mud or water, is then pumped behind the cement slurry in order to force the cement up into the annulus between the exterior of the casing and the borehole. The cement slurry is then allowed to set and harden to hold the casing in place.
Successful cementing of the well pipe and casing during oil and gas well completion requires the cement slurry to be characterized by several important properties. For instance, the cement slurry must have a pumpable viscosity, fluid loss control, minimized settling of particles and the ability to set within a practical time.
Under normal conditions, hydraulic cements, such as Portland cement, quickly develop compressive strength upon introduction to a subterranean formation, typically within 48 hours from introduction. As time progresses, the cement develops greater strength while hydration continues. It is sometimes necessary to use a lightweight cement when there is a weak subterranean formation. A lightweight cement exerts a lower hydrostatic pressure on the formation than do regular cements.
When used, lightweight cements should exhibit a low density without sacrificing strength. Density can be lowered in such cements by mixing a lightweight additive, such as sealed microballoons, with the cement. U.S. Pat. No. 6,832,652 discloses the use of glass or ceramic microspheres, such as borasilicate glass, to render a cement of low density, typically ranging from about 13.0 pounds per gallon (ppg) to about 6.0 ppg or less. In some applications, the use of such lightweight additives is economically unfeasible or is procedurally cumbersome.
Lightweight cements may also be created by injecting the cement with a gas, such as nitrogen, in order to create a foam. This foaming operation can be difficult because additional nitrogen equipment is required and the addition of gas must occur on-site which requires that the density of the slurry be carefully measured before use.
Alternatives have therefore been sought for lightweight cements which render the requisite compressive strength and yet are not foams and do not require lightweight cement additives.